Non-emissive flat panel displays, including liquid crystal displays, are commonly used in portable computer systems, televisions and other electronic display devices. Non-emissive flat panel displays can be divided into two types--reflective and transmissive.
Reflective type displays use ambient light for energy efficiency. However, lacking a suitable built-in lighting system and suffering from a low brightness, the reflective type displays are used mainly for low end monochromatic applications. Back-lit transmissive type displays, on the other hand, are used mainly in high end products such as notebook computers and televisions because they beneficially have a very high contrast ratio and low surface reflectivity.
One disadvantage to the backlighting system of a transmissive display is that it consumes a significant amount of power and its brightness is limited by available lighting power. Most of the transmissive type displays are not suitable for outdoor applications because they lack sufficient brightness.
Transmissive type displays completely rely on their build-in lamps for illumination. As a result, the battery life of the portable computers are quite short (usually, less than 4 hours). Furthermore, most of the existing transmissive type displays do not have a sufficient brightness for outdoor applications. Moreover, since transmissive type displays using backlighting systems of the prior art are not transparent, ambient light cannot be used to enhance their illumination. There is a need to obtain a sufficiently bright transmissive type display which uses as little power consumption as possible while also generating a sufficient brightness for outdoor use.
Overhead projection palettes which are used in conjunction with overhead projectors, are used to transfer data on a computer screen to a projection screen. These are transmissive type displays which rely solely on ambient light for their illumination, specifically, an overhead projector having capable of high lumen output.